1. Field of the Invention
The invention concerns a method of transmitting in ATM cells information supplied in the form of a series of separate, identically structured entities each corresponding to an independently usable whole for a given application. It also concerns devices for implementing this method.
2. Description of the Prior Art
The method is more particularly intended for use in a broadband integrated services digital network, in the adaptation layer between the layer corresponding to the asynchronous transfer mode (ATM) network and the top layer corresponding to the application concerned (reference model defined in CCITT Recommendation 1.321).
The cells used to transmit the entities are preferably ATM cells as defined in CCITT Recommendation 1.361.
The information to be transmitted from a source application is in analog form and can be organized into a series of entities with the same structure in which each entity represents a whole independently usable by the application to which the information is sent.
The entities of a series from the same source application are transmitted in succession in order to enable the flow of information from they originate to be reconstituted; they must therefore be receivable at a clock rate which is at least relatively regular, i.e. possibly subject to some degree of jitter.
In practise these entities are modular units of information, for example audio or video information, the respective sizes of which may vary.
In the case of audio information, for example, an entity represents a set of digital data representing selected energy components calculated from a group of "s" successive samples of a stream of audio information.
In the case of video information, for example, an entity represents a set of digital data representing an image to a specific format.
The temporal stringing of the entities is usually a service interpreted at the destination.
As already mentioned, the throughput of information transmitted by the entities can vary, especially when (as in the above example) the original data relating to each entity undergoes quantizing and statistical coding prior to transmission.
As mentioned above, it is essential that, as far as possible, every entity sent can be recovered after ATM transmission from its source to its destination. When ATM is used, errors can occur that are specific to the transmission of entities by means of cells, in addition to known errors that may occur at the level of the data bits which form the cells. There is therefore the risk of the network sending a cell somewhere other than its real destination in the event of an error in that part of the cell defining its destination that is either undetected or is not corrected properly.
The invention therefore proposes a method of transmitting in ATM cells analog information supplied in the form of a series of separate, identically structured entities each representing a whole independently usable by a given application, the method being intended to offer a high quality of service to the user, in particular by enabling good detection and good correction of transmission errors and elimination of entities that cannot be recovered on reception.
Each cell includes a header in which its destination is defined and an information field, each of which has a respective standard size.